Line echoes, or hybrid echoes, naturally occur in telecommunications networks wherein a portion of the transmission path consists of a two wire segment. Typically two wire segments are found between telephone company offices and the customer or subscriber. Echo occurs, therefore, on most all typical telephone calls. This echo is not perceived as such by the participants unless the time delay between the participants is greater than approximately 10 ms. Since nearly all local calls have a delay less than 10 ms, echo is perceived as a problem primarily on long distance calls. In such calls, the delay between two talkers may range from 10 ms to over 250 ms, if a geostationary satellite is involved. In this latter case, the round trip delay is the sum of the delay in both directions and would be over 500 ms. When echo is present, the speakers hear their own voice with a 500 ms delay. This can generally be quite disruptive and disconcerting.
On long distance transmission paths, echo cancellers are normally employed, one at each end. The typical operation of conventional echo cancellers using adaptive filters can be found in a number of references, such as: U.S. Pat. No. 3,789,165 to Campanella et al.; U.S. Pat. No. 4,031,338 to Campanella et al. and U.S. Pat. No. 5,664,011 to Crochiere et al. In a transversal filter of the prior art, the echo generated by the two wire transmission path may be effectively estimated and subtracted from the send direction signal, thus providing a nearly echo free signal. Effective estimation and subtraction is commonly referred to as convergence, as the estimated echo cancellation signal converges toward a close replication of the echo signal. A principal problem with this approach is that sometimes the echo of the far end signal can be and typically is combined with the signal of the speaker at the near end. The situation is called double talk when both the near end and far end speaker are speaking at the same time. In this case, the adaptive estimation procedure can become confused and may diverge temporarily, causing increased echo.
Ochiai et al. “Echo Canceller with Two Echo Models” IEE Transactions on Communication vol. COM-25, No. 6 June 1977, pp. 589–595, proposed a dual filter approach, as illustrated in FIG. 1, wherein one filter 18, a non-adaptive filter, is used to cancel the echo. Since filter 18 is never adapted, filter 18 is never disturbed by double talk. A second filter 16 is adapted. If the adaptive second filter 16 is able to estimate and reduce the echo better than the first filter 18, then the non-adaptive filter's coefficients are is replaced by those used by the adaptive filter 16. If however, the adaptive filter 16 diverges due to double talk, the non-adaptive filter's coefficients will not be replaced. This provides an excellent immunity to the ravaging effects of double talk. However, there are two drawbacks. The first is that the complexity of the echo canceller is now considerable higher. Hence it is prohibitively expensive. Secondly, since the adaptive filter 16 must find an improved solution before its results can be used in the non-adaptive filter 18, the convergence time, or time taken to find the reflection coefficients can be unacceptably slow.